Blood volume measurements in the neonatal lamb: validation of a method using [51Cr]-labelled red cells.

1. The reproducibility of blood volume measurements, using [51Cr]-labelled red blood cells (RBC) was tested in three lambs, 79-89 days of age, and the accuracy was tested in eight lambs, 83 +/- 0.4 days of age, in which blood volume was measured simultaneously by [51Cr]-RBC and [125I]-gamma globulin plus haematocrit (hct). 2. The blood volumes (mean +/- s.e.) of the three lambs, on four occasions, were 64.5 +/- 2.5, 68.8 +/- 2.9 and 63.9 +/- 3.7 mL/kg, respectively, and the coefficients of variation were 7.8, 8.6 and 11.6%, respectively. 3. The mean blood volume of eight lambs was 60.9 +/- 3.3 mL/kg by [51Cr]-RBC, and 60.8 +/- 2.4 mL/kg from plasma volume and hct. These were not statistically different. 4. Arterial hct was 31.2 +/- 0.5%, which was not statistically different from the whole body hct of 31.2 +/- 2.6%. 5. It is concluded that [51Cr]-RBC can be used to measure accurate and reproducible blood volumes in lambs.